1. Field of the Invention
This invention relates to a method and apparatus for subterranean drilling. In one aspect, this invention relates to a method and apparatus for drilling in relatively hard subterranean formations, such as granite, limestone, sandstone and other rock formations as well as other materials such as concrete. In another aspect, this invention relates to a method and apparatus for horizontal subterranean drilling. In another aspect, this invention relates to a method and apparatus for performance of reworking operations in well bores.
2. Description of Related Art
Conventional subterranean drilling typically is performed using a rotary drill bit attached to a drill string. A drill string, which is normally associated with an oil well rig, is a column, or string, of drill pipe, or coiled tubing that transmits drilling fluid by means of one or more mud pumps and rotational power by means of a kelly drive or top drive or downhole motor to the drill bit. The drill string is hollow so that the drilling fluid can be pumped down to the bottom or end of a borehole through the interior of the string and circulated back up through the annulus formed between the drill string and the borehole wall. The drill string is typically made up of four sections: 1) bottom hole assembly; 2) transition pipe, also referred to as heavy weight drill collar; 3) drill pipe; and 4) drill stem subs. The bottom hole assembly typically comprises a drill bit which is used to break-up the rock formations and may also include other components such as a downhole motor, rotary steerable system, measurement while drilling (MWD), and logging while drilling (LWD) tools.
A heavyweight drill collar is used to provide a transition between the drill bit and drill pipe. This helps to reduce the number of fatigue failures seen directly above the bottom hole assembly. Drill pipe makes up the majority of a drill string, which may be up to 15,000 feet in length for an oil or gas well vertically drilled onshore in the United States and may extend to over 30,000 feet for an offshore deviated well. Drill stem subs are used to connect drill string elements.
A relatively modern drilling technique involves using coiled tubing instead of conventional drill pipe. Coiled tubing is metal piping which comes spooled on a large reel. This has the advantage of requiring less effort to trip in and out of the borehole (the coil can simply be run in and pulled out of the borehole while drill pipe must be assembled and dismantled joint by joint while tripping in and out). Instead of rotating the drill bit by using a rotary table or top drive at the surface, it is typically turned hydraulically by a downhole motor, powered by the motion of drilling fluid pumped from the surface. One of the benefits, as well as disadvantages, of coiled tubing is its flexibility, which facilitates directional drilling, but which also reduces the amount of force that can be applied to the drill bit when encountering hard underground formations and when drilling non-vertical boreholes.
The drill bit, which is one of the components of the bottom hole assembly, is typically made of alloy steel and comprises pieces of carbide or diamond cutting surfaces to break the hard material of the subterranean formation. The two most common types of drill bits are fixed cutter bits, which use polycrystalline diamond compact cutters to shear rock with a continuous scraping motion, and roller cone bits, which comprise teeth on wheels which turn as the drill string is rotated, thereby applying a crushing pressure to the rock, breaking it up into small pieces.
In most subterranean drilling applications, especially when drilling into harder materials, it is necessary to apply a certain amount of force on the drill bit to achieve the desired drilling speed. In vertical boreholes, when using a substantially rigid drill string, the force on the drill bit is controlled by the weight of the drill string above the drill bit. However, this method becomes less effective when drilling non-vertical or curved boreholes and even less effective when using a drill string of coiled tubing. Methods have been developed to improve the drilling speed in these applications, the most common of which involves the use of a tractor which anchors to the surface of the drilled borehole above the bit while the downstream drill string is powered forward using electrical or hydraulic force. Although effective in many instances, these tractor systems are expensive and have difficulties in maneuvering through softer formations where the surface breaks down and, thus, do not provide the needed anchoring.